In practice, the external device is most often an additional antenna that improves the quality of the radio connection, and the radio device is most often a mobile phone. An additional antenna may be needed in a vehicle, for example, when the field strength of a base station within the body of the vehicle is low. The additional antenna is then naturally outside the body, fastened to it. The vehicle can have a fixed holder for using an external antenna, and a phone placed in it is coupled to the external antenna by a coupling part and a cable. The external device can also be a measurement device or analyzer used in production or servicing. Naturally, such a device does not radiate electromagnetic energy, although the radio device “sees” it as a mere antenna.
For connecting an additional antenna, a mobile station can have a coaxial connector coupled to the antenna port. However, such a connector arrangement based on galvanic coupling is relatively expensive and unreliable in the course of time. Therefore, electromagnetic coupling can be used instead of galvanic coupling. FIG. 1 shows a known arrangement for coupling to the antenna of a radio device as two partial images. The partial image on the left shows the radio device RD as seen from the back and the other partial image shows it as a simplified longitudinal section. The antenna of the radio device is an internal antenna of the PIFA type (planar inverted F-antenna). The longitudinal sectional image thereof shows a ground plane GND, a radiating plane 121 and a short-circuit conductor 125. The radiating plane is relatively close to the back cover 105 of the radio device, and the ground plane is inner within the device. The arrangement includes a coupler 110 having approximately the spread of the internal antenna and being fastened with adhesive joint, for example, at the antenna on the outer surface of the back cover 105. The coupler includes a conductive strip-like coupling element 111, which has, for the above mentioned reason, relatively strong electromagnetic coupling with the radiating plane 121. Through this coupling, radio frequency energy is conveyed from the near field of the antenna outside the radio device and vice versa. A coaxial intermediate cable 150 runs from the coupling element to the additional antenna or other external device.
In this description and the claims, the short expression “coupler” means a device to be fastened to a radio device without modifying it mechanically, through which a radio frequency connection is arranged to an external device.
The applicant is aware of the method according to FIG. 2, described in patent application Ser. No. FI 20022117, for coupling to the internal antenna of a mobile phone, for example, for connecting an additional antenna. The coupling arrangement includes a coupler 210 placed on the back cover of the phone, having a first and a second coupling part. The first coupling part is intended to be placed at the internal planar antenna of the phone, and it has two interconnected coupling conductors 211 and 212, which are rigid conducting wires in this example. Of the phone, only the radiating plane 221 of the planar antenna is drawn in the main FIG. 2. The small side image also shows a part of the phone casing 205 between the radiating plane and the coupling conductor 211. In this example, the radiating plane is divided into a first and a second branch for increasing the number of operating bands. The feed and short-circuit points of the antenna are in an area in which the branches are interconnected. When the coupler has been put in its place, the coupling conductor 211 is located on top of the first branch as viewed from the direction of the normal to the radiating plane, and the coupling conductor 212 is correspondingly located on top of the second branch. In this way, the coupler is made to function both on the lower and upper band of the antenna.
The second coupling part of the coupler is intended to be located at the internal ground plane of the phone, and it consists of a conductive plate 213 with its side edges bent to form a right angle. These side edges press against the sides of the phone so that the coupler is fastened to the phone. There is a significant electromagnetic coupling between the conductive plate 213 and the ground plane. The coaxial intermediate cable 250 between the coupler and am external device is fastened to the conductive plate. The inner conductor of the intermediate cable is galvanically coupled to the interconnection point of the coupling conductors 211 and 212, and the outer conductor is galvanically coupled to the conductive plate close to the coupling point of the inner conductor. Through the coupling between the conductive plate and the ground plane, energy from the radio frequency field of the ground plane can also be transferred to the external device. The conductive plate 213 and the conducting wires 211, 212 function as a generator feeding the additional antenna, for example, through the intermediate cable 250.